1. Field of the Invention
The invention relates to the field of capacitors, and more particularly, to an improved coating for use between a foil-type electrode and a plate of a solid dielectric capacitor.
2. Description of the Prior Art
Solid dielectric capacitors generally comprise one or more thin sheets of solid dielectric material, such as mica, having an electrically conductive material applied on one or both sides thereof by various well-known techniques, such as spraying or silk screening. Typically, the conductive material is a silver-based paint. The coated mica plates are stacked one atop another, with thin conductive foil electrodes interleaved therebetween to form the capacitor. The conductive coating on the dielectric sheets acts as one of the plates of the capacitor, while the foil electrodes are used to provide connection with external terminals. The entire package may be encapsulated or otherwise sealed in a well-known manner to form the finished capacitor. Representative examples of such capacitor construction are shown in U.S. Pat. Nos. 3,718,844, 3,229,173, 2,935,669, and 2,522,713.
One problem that arises in manufacturing solid dielectric capacitors having foil electrodes is that some means must be provided for securing the alternating layers of dielectric and foil electrodes together. The most prevalent and economical way of doing this is to interleave the foil electrodes and sheets of solid dielectric material and then impregnating the stacked sections with a bonding agent, such as by dipping the stacked sections into the bonding agent. The bonding agent can be, for example, wax, varnish, epoxy, or other types of adhesive.
Alternatively, the bonding agent can be first applied to the foil electrodes by various well-known means such as spraying, dipping, etc. The coated foil electrodes and dielectric sheets are then laminated together, typically by a combination of heating and pressure which acts to squeeze out excess bonding agent and to bond the various components together into a single unit.
One drawback to the use of such bonding agents is that they typically are insulators which, if improperly applied or cured, can remain in sufficient thickness and quantity on the foil electrode to prevent good electrical contact from occurring between the electrodes and the adjacent capacitor plates. Such a condition increases the electrode resistance and reduces the performance of the capacitor, especially at very low voltages.